L-Histidine by Twinlab is an amino acid. Twinlab L-Histidine helps repair tissues and is a good for rheumatoid arthritis, anemia, and allergies. Histidine is called a semi-essential amino acid (protein building block) because adults generally produce adequate amounts but children may not. Histidine is also a precursor of histamine, a compound released by immune system cells during an allergic reaction.